1. Field of the Invention
The invention relates to an intelligent control method for vacuum pumps used in an industrial furnace complex, particularly in vacuum furnace systems. The invention also relates to an industrial furnace complex associated therewith.
2. Description of the Related Art
According to DE 10152204 B4, the stated object is to improve a vacuum gas carburising complex so as to avoid the drawbacks of not being able to monitor and record data relating to a carburising atmosphere during the carburising process.
In this context, the following steps are carried out in a method for measuring and/or regulating the carburising atmosphere inside a carburising chamber of a vacuum gas carburising system:                1.: Introducing workpieces into the carburising chamber;        2.: Creating and maintaining a vacuum in the carburising chamber;        3.: Introducing a carburising gas into the carburising chamber;        4.: Measuring the atmosphere in the carburising chamber with a vacuum-tight oxygen probe.        
With this method for controlling the process gas, it is only possible to measure the current state of the furnace atmosphere regularly during the carburising phase. However, with this method it is not possible to engage or disable the vacuum pumps throughout the entire process according to the status of the process concerned.
An automated system for monitoring operational safety and controlling the process cycle in a vacuum heat treatment furnace is also known from DE 41 21 277 C2, wherein a pressure sensor for measuring the pressure in the furnace housing and at least one gas sensor arranged in the immediate vicinity of the furnace are provided. Each sensor cooperates with a separate evaluation unit to initiate a safety program if a predetermined pressure is not reached in the interior of the housing and at the same time a given gas concentration is detected in the area surrounding the furnace. This immediately causes the cooling gas inlet valve to be closed, the gas outlet valve to be opened and a purge gas inlet valve to be opened. This safety program is activated in a line that connects a purge gas reservoir with the interior of the furnace housing. The pressure in a housing interior chamber and in the area surrounding the furnace is equalised depending on the coolant gas concentration at the gas outlet valve as recorded by a gas sensor that is integrated in a branch line to the gas outlet line.
This additional safety device in a vacuum furnace with high pressure hydrogen quenching guarantees that the furnace is flushed with an inert gas if a hydrogen leak is detected and a combustible/explosive gas mixture cannot be formed. However, it does not exercise any corresponding control/regulation of the vacuum pump.
A technical search for solution options must also include an examination of the device described in EP 0524 368 B1 (similar to DE 41 21 277 C2). In this case too, automated monitoring and control of the process sequence is provided in a vacuum heat treatment furnace, particularly a furnace that is operated for the purpose of annealing metal workpieces with hydrogen under overpressure as the coolant gas. In this case, a vacuum pump is connected to housing. Gas inlet and gas outlet apertures open into the heating chamber. The system includes a motor-fan unit, a coolant gas reservoir, a heater unit and a heat exchanger in the coolant gas circuit as well as a pressure sensor measuring the pressure in the furnace housing and at least one gas sensor arranged in the immediate vicinity of the furnace, and each of these components cooperates with an evaluation unit to initiate a safety program if a predetermined pressure is not reached in the interior of the housing and at the same time a given gas concentration is detected in the area surrounding the furnace.
As described in the preceding, this program immediately causes the coolant gas inlet valve to be closed and the gas outlet valve and a purge gas inlet valve to be opened. The purge gas inlet valve is integrated in a line that connects purge gas reservoir with an interior chamber of the furnace housing and so ultimately brings about pressure equalisation between the housing interior and area surrounding the furnace depending on the coolant gas concentration registered at the gas outlet valve by a gas sensor integrated in a branch line to the gas outlet line.
Accordingly, this solution returns full circle to the document DE 41 21 277 C2 examine previously with an additional safety device for a vacuum furnace with high pressure hydrogen quenching which ensures that the furnace is flushed with an inert gas if a hydrogen leak is measured, thus guaranteeing that a combustible or explosive gas mixture cannot form.
Accordingly, the general process sequence represented in both patents also does not disclose how the vacuum pump might be controlled or regulated.
A method for regulating the vacuum in a chamber that is connected to a pump device comprising several pumps arranged in series is disclosed in DE 100 43 783 A1. In this context, at least one suction parameter is modified depending on both the high vacuum pressure present in the chamber and a predefined setpoint pressure. The change to the suction parameter is effected by using at least one regulating parameter, the regulating parameter being determined according to the high vacuum pressure that is present inside the chamber.
While this regulation of the suction power of a pump is economical in terms of energy consumption, it is not able to isolate the pumps from the recipient by means of a valve if overpressure phases occur in the vacuum furnace during high pressure gas quenching or convection operation for example.
A diaphragm or piston pump, or a combined diaphragm/piston pump with a device for pressure-dependent reduction of the suction chamber expansion speed is also described in DE 198 16 241 C1. In this device, if the aspiration pressure or control signal falls below a certain value the speed of the drive unit is adjusted automatically, the pressure or control signal and speed value pairs in this range are measured and stored, and from this is calculated the minimum suction value with which the lowest final vacuum can be achieved. On this basis, the speed of the drive unit is set to the associated optimum speed.
The disadvantage of such economical regulation of a pump's suction power by varying its suction or speed is also that the system cannot determine when the pumps are isolated from the recipient by a valve. However, this is essential in the overpressure phases that occur in a vacuum furnace (high pressure gas quenching or convection operation).
DE 699 07 890 T2 also discloses a method and device for regulating pressure in vacuum systems, in which the following steps are carried out:                Step 1: Reading a value for a desired pressure from an electronic memory that reflects a desired pressure level for the process chamber;        Step 2: Reading a value for a desired gas flow from the electronic memory that represents a desired flow of gases through the process chamber;        Step 3: Setting a choke valve in an initial position, wherein the choke valve is used to regulate the pressure in the process chamber;        Step 4: Measuring the pressure in the process chamber;        Step 5: Calculating a difference between the desired pressure and the measured pressure, and        Step 6: Following the adjustment step, readjusting the choke valve at least once on the basis of the difference between the desired pressure and the measured pressure.        
In this context, the readjustment is carried out using a proportional and integral regulator and the delay thereof for a specified period.
With this method and device for pressure regulation in vacuum systems, it is possible to regulate the suction power, but the “intelligence” of the system is limited by the fact that a switch-off function is not triggered in the process phases that are performed without a vacuum.
This shortcoming (no switch-off in the phases of the process that do not require a vacuum) is also not corrected by the solution suggested in JP 2008002274 A, which discloses an evacuation device for regulating the furnace pressure in the vacuum furnace by altering the speed of the vacuum pump. In order to carry out the evacuation, a first recirculation line between the pump outlet and the pump inlet is opened when the lowest speed for the vacuum pump is reached. A second recirculation line is opened if the lowest speed for the vacuum pump is reached again despite the first recirculation line being open. When the upper limit of the pump speed regulator is reached, the recirculation lines are closed. The recirculation lines are opened and closed by corresponding valves. The recirculation flow is adjusted using resistors.
A brief review of a different field such as is represented by JP 2001214868 A gives no hint as to how to ensure that the system switches off during the process phases where a vacuum does not occur. In this case, only one vacuum control unit in one furnace is provided for simply adapting the vacuum level in the furnace without increasing the size of the device and raising the vacuum pump output while the vacuum arc furnace is operating. In this case, the vacuum arc furnace is equipped with a vacuum pump that is connected to the interior of the furnace via a suction line. A vacuum atmosphere is created in the furnace housing with the shared use of a mechanical booster pump and a rotary pump. When the smelting furnace is in use, the rotating speed of the pumps is controlled by an inverter. When the suction power is altered, the vacuum is adjusted in the furnace. When the vacuum display signal reaches the controller, the inverter automatically initiates control of the pump speed.
A further consideration of U.S. Pat. No. 3,736,360 instructs one skilled in the art with control system for an electrically heated vacuum furnace. Among other devices, this system is equipped with a vacuum pump that is connected to the interior of the furnace via a suction line, and also with a plurality of heating elements for various zones and with a temperature measuring device for each zone, and with a controlled measuring instrument for the master zone. These instruments are connected to a primary furnace regulator and secondary regulators for the individual zone. A control valve in the vacuum line or a controlled cock, connected to the vacuum line, make it possible to keep the pressure in the furnace constant while the pump operates at constant speed. A pressure gauge connected to the vacuum line is also connected to the heater elements.
After evaluating this intelligent system for regulating the suction power, no provision was found for a switch-off capability for the phases without vacuum.
Based on a general survey of the evaluated prior art and practical experience in process operation of vacuum systems, “emergency devices” and “pressure regulating devices” are known that primarily use safety or pressure as a “regulating parameter. But more stringent requirements are applied to vacuum heat treatment in a modern vacuum furnace to ensure that besides the existing temperature-time sequences it is able to respond flexibly in anticipation of the pressure-time sequences.
In view of the above, it may be concluded that the problem of obtaining information about when the vacuum pumps can be shut off entirely after a certain point in time while process gas is being evacuated at certain times in a given process has not yet been solved.